As regards a lead-free optical glass having a high refractive index and a high dispersion region, a phosphate system has been proposed, for example, in Patent Documents 1 and 2, but almost none have been specifically proposed for a composition having a refractive index (nd) of more than 1.98.
A higher refractive index (nd) enables obtaining a lens of larger power and is preferred, nevertheless, as for an optical glass having a high refractive index of more than 1.98, only a few phosphate systems free of lead or tellurium which are an environmentally hazardous substance have been proposed in Patent Documents 3 and 4.
Patent Document 3 proposes a lead-free phosphate optical glass having a high refractive index and a low Abbe's number, where, however, the composition satisfying the condition of nd being 1.98 or more has as high a liquidus temperature (LT) as 920° C. or more and may be insufficient in the devitrification resistance.
Furthermore, the optical glass proposed in Patent Documents 3 and 4 is a glass containing large amounts of Ti, W, Bi and the like that affect the absorption in the visible region due to a change in the valence, and if the liquidus temperature (LT) is excessively high, the glass cannot be sufficiently retained at a low temperature, and a melt in a redox state before performing plate formation or gob molding by discharging the glass cannot be controlled to an oxidation state advantageous in enhancing the transmittance of glass (with Ti, W or Bi, when the valence is high, the degree of coloration is small and in turn, the transmittance is high) and is liable to remain in a reduction state giving a bad transmittance, as a result, a high transmission characteristic important as an optical glass cannot be satisfied.    Patent Document 1: JP-A-2003-321245    Patent Document 2: JP-A-2005-8518    Patent Document 3: JP-A-2007-15904    Patent Document 4: JP-A-2006-111499